Impact of diurnal temperature fluctuations on larval settlement and growth of the reef coral Pocillopora damicornis

Diurnal fluctuations in seawater temperature are ubiquitous on tropical reef flats. However, the effects of such dynamic temperature variations on the early stages of corals are poorly understood. In this study, we investigated the responses of larvae and new recruits of Pocillopora damicornis to two constant temperature treatments (29 and 31 degrees C) and two diurnally fluctuating treatments (28-31 and 30-33 degrees C with daily means of 29 and 31 degrees C, respectively) simulating the 3 degrees C diel oscillations at 3m depth on the Luhuitou fringing reef (Sanya, China). Results showed that the thermal stress on settlement at 31 degrees C was almost negated by the fluctuating treatment. Further, neither elevated temperature nor temperature fluctuations caused bleaching responses in recruits, while the maximum excitation pressure over photosystem II (PSII) was reduced under fluctuating temperatures. Although early growth and development were highly stimulated at 31 degrees C, oscillations of 3 degrees C had little effects on budding and lateral growth at either mean temperature. Nevertheless, daytime encounters with the maximum temperature of 33 degrees C in fluctuating 31 degrees C elicited a notable reduction in calcification compared to constant 31 degrees C. These results underscore the complexity of the effects caused by diel temperature fluctuations on early stages of corals and suggest that ecologically relevant temperature variability could buffer warming stress on larval settlement and dampen the positive effects of increased temperatures on coral growth.